The cyclic triamine 1,5,9-triazacyclododecane (L), C9H21N3, reacts with Mo(CO)6 in Decalin at 150 °C to yield yellow, air-stable LMo(CO)3 (1). Oxidation of I with bromine, nitrous acid (or NOBF4), or hydrochloric acid in the presence of 02 affords the monomeric complexes [LMo(CO)3Br](Br3), [LMo(CO)2NO]BF4, and LMoCl3, respectively. Oxidative decarbonylation of 1 in HC104 with 02 gives the two isomers purple anti-[L2Mo204] (C104)2-2H20 and yellow syn-[L2Mo204](C104)2-H20. Reduction of the former with zinc amalgam in aqueous solution in the presence and absence of coordinating acids (HBF4, formic acid, acetic acid, hydrochloric acid) affords diamagnetic, green, dimeric bis(g-hydroxo)-bridged species of molybdenum(III): [L2Mo2(g-OH)2(OH)2]I2-2H20, [L2Mo2(g-OH)2(g-HCOO)]I3-2H20, [L2Mo2(g-OH)2(g-CH3C02)]I3, and [L2Mo2(g-OH)2Cl2](C104)2. Purple anti-[L2Mo204](C104)2-2H20 crystallizes in the monoclinic space group P2¡/c with a = 8.515 (8) Á, b = 11.11 (2) Á, c = 16.56 (3) A, ß = 92.3 (1)°, V = 1565 (1) Á1 23, and d^a¡ = 1.77 g/cm3 for Z = 2 and , = 833.4. Least-squares refinement of the structure based on 3216 observations led to final discrepancy indices of R = 0.039 and R" = 0.045. The structure consists of dimeric cations with an iznZ/-dioxobis(g-oxo)dimolybdenum(V) core (edge-sharing octahedra); the short Mo-Mo distance of 2.586 (1) Á and the diamagnetism of the compound indicate a Mo-Mo single bond. The four-membered Mo202 ring is planar.In alkaline aqueous solutions the purple aníz-[L2Mo204]2+ complex isomerizes irreversibly to the yellow syn isomer. The kinetics of this reaction have been measured, and a possible mechanism is discussed. This isomerization has also been demonstrated by using 95Mo NMR spectroscopy.